ABSTRACT Autism Spectrum Disorder (ASD) is a severe, life-long neurodevelopmental disorder that is associated with considerable impairment for individuals and a substantial burden to their families and communities. Little is known about the causes or correlates of ASD. While diagnostic practices are improving ASD identification, much remains to be discovered about risk and protective factors for ASD and about the phenotypic variation and prevalence of co-morbid conditions. In response to growing concerns, the Children's Health Act of 2000 mandated CDC to establish ASD surveillance and research programs that address the magnitude, incidence, and causes of ASD and related developmental disabilities. The Centers for Autism and Developmental Disabilities Research and Epidemiology (CADDREs) were established at six national sites (California, Colorado, Georgia, Maryland, North Carolina, and Pennsylvania) to fulfill this mandate and are currently carrying out the second phase of the Study to Explore Early Development (SEED), a population-based case- control study. SEED addresses hypotheses including: ASD phenotypic variation, including clustering of core symptoms, cognitive status, and presence of co-morbidities; gastrointestinal features; genetic variation and gene-environment interaction (GxE); infection, immune function, and autoimmunity factors; and hormonal factors and maternal reproductive characteristics. Data collection includes developmental assessments and pre- and perinatal health and environment via interviews, medical record review, self-administered questionnaires, and biologic samples. As of December 2015, SEED 1 & 2 had completed data collection on 4652 families - 1341 with ASD, 1722 with other developmental disorders, and 1589 controls. In this SEED 3 proposal, North Carolina SEED proposes new recruitment and data collection for 125 children in each group, to contribute to the Network total of 625 children per SEED 3 study group. This increased, combined sample size will enable well-powered assessment of SEED hypotheses, particularly for phenotypic subgroups and GxE interactions. Given the experience of SEED 1 & 2 and the infrastructure in place at each CADDRE site, SEED 3 can be quickly implemented, creating a combined SEED sample of clinical, risk factor, and biological specimens and data on over 6500 families. SEED would be the largest study of ASD of this kind, making significant contributions to our understanding of the complex autism phenotype and identifying potential risk and protective factors for ASD.